I think your last point holds true, but it's really not an equivalent amount of stress you're putting on your body in an office. You can always go outside more after work and remain physically active to counteract the damage you're doing by sitting in an office all day, but if you're in a physically intensive trade, you can't avoid the damage you're doing by remaining less active after work.

Also you're going to have less energy to do the physical activity you want to do.

I paved roads for a summer at 18 and even though I was a very young energetic kid, I was barely getting anything out of my evening workouts or soccer games with friends after 10 hours outside shoveling asphalt.

My dad passed away a couple of years ago of some weird cancer after a career working in nuclear power (we got a class action settlement from the DOE).

There are definitely some careers that are more dangerous than others.

Sure I might get some RSI, but my welder friend has to contend with someone on site accidentally creating phosgene gas. I don't feel like the two levels of potential self harm are in the same ball park.

Your welder friend could also end up with RSI- depending on how much grinding he does in a day. That was the main reason I quit being a mechanic, it's actually easier to prevent RSI in an office environment by careful selection of equipment, whereas running a grinder for an hour with your wrists at a funny angle since it's the only way to get the tool into the work is sometimes just required.

Office dangers: carpal tunnel, rounded shoulders, and undesirable weight gain to name a few.

Sure but you don't get carpal tunnel from screwing up once, you get it by screwing up over and over for a decade. Trade injuries are often one shots. Being said if you don't have the discipline to avoid posture injuries you might not have what it takes to survive a trade.

Never mind that work conditions have improved since the days of GP’s grandpa.

Plumbers pick their own work conditions as they are often self employed. So uh kinda? Mostly not. Heavy things are still heavy and still need to be lifted. You're probably not going to bring in a jack.

My best friend was a HVAC repairman and is a programmer now. He's happier as a programmer. Some people are happier as an HVAC repairman, and more power to them. However it's not the norm for people who have done both.

Plumbing isn't the only kind of blue collar work, but I expect even there the work safety has improved considerably. Many things that used to be heavy are now light (copper and lead vs PVC and other plastics), many things that used to be toxic are no longer required or have safer alternatives (e.g., solders), and in general the cultural expectation for a plumber to risk life or limb for a job is far lower.

And if you're working in a factory, conditions have improved far more drastically. Worker safety is serious business given OSHA and legal liability associated with workplace injury.

Happy to hear about your best friend; I doubt it extrapolates, but it doesn't matter much to me--I don't have a dog in the fight.

Sounds like you should spend some time in a trade. It's still worth doing don't get me wrong but there are more risks, they are harder to avoid, and with worse consequences.